On Mon, Aug 15, 2022 at 8:02 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > On Mon, Aug 15, 2022 at 07:45:16AM -0700, Alexei Starovoitov wrote: > > On Mon, Aug 15, 2022 at 7:33 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > > > It is in full control of the 'call __fentry__'. Absolute full NAK on you > > > trying to make it otherwise. > > > > Don't mix 'call fentry' generated by the compiler with nop5 inserted > > by macroses or JITs. > > Looking at: > > https://lore.kernel.org/all/20191211123017.13212-3-bjorn.topel@xxxxxxxxx/ > > this seems to want to prod at the __fentry__ sites. > > > > > Soon we will have nop5 in the middle of the function. > > > > ftrace must not touch it. > > > > > > How are you generating that NOP and what for? > > > > We're generating nop5-s in JITed code to further > > attach to. > > Ftrace doesn't know about those; so how can it break that? > > Likewise it doesn't know about the static_branch/static_call NOPs and > nothing is broken. > > Ftrace only knows about the __fentry__ sites, and it *does* own them. > Are you saying ftrace is writing to a code location not a __fentry__ > site? Let's keep it in one thread: > It wasn't long before. Yes it landed a few months prior to the > static_call work, but the whole static_call thing was in progress for a > long long time. > > Anyway, yes it is different. But it's still very much broken. You simply > cannot step on __fentry__ sites like that. Ask yourself: should static_call patching logic go through ftrace infra ? No. Right? static_call has nothing to do with ftrace (function tracing). Same thing here. bpf dispatching logic is nothing to do with function tracing. In this case bpf_dispatcher_xdp_func is a placeholder written C. If it was written in asm, fentry recording wouldn't have known about it. And that's more or less what Jiri patch is doing. It's hiding a fake function from ftrace, since it's not a function and ftrace infra shouldn't show it tracing logs. In other words it's a _notrace_ function with nop5.
